1. Field of the Invention
This invention relates to a release agent comprising organosilicon compounds. More particularly, this invention relates to a release agent based on an amino-functional diorganopolysiloxane.
2. Background Information
The utility of amino-functional diorganopolysiloxanes as paint additives that provide releasability and prevent blocking is disclosed in Japanese Patent Publication Number Sho 53-3992 [3,992/1978].
The utility of amino-containing diorganopolysiloxanes as release agents for the thermoplastic resin toner in color xerographic copiers is described in Japanese Laid Open Patent Application [Kokai or Unexamined]Number Hei 4-230784 [230,784/1992]); the use of these diorganopolysiloxanes as stress relaxers or release-improving additives for epoxy resins used as semiconductor sealants is described in Japanese Patent Application Laid Open Number Hei 3-275710 [275,710/1991]. Japanese Patent Application Laid Open Number Sho 60-229719 [229,719/1985] teaches using these diorganosiloxanes as lubricating release agents for tire-molding bladders
The amino-functional dimethylpolysiloxanes described in the aforementioned laid open Japanese patent applications contain large quantities, typically from 50,000 to 200,000 ppm, of dimethylsiloxane oligomers containing up to 20 silicon atoms. During long-term use at elevated temperatures, these oligomers evaporate and stick to machinery such as exhaust fans, causing the formation of oily stains. In addition, this oil can drip onto and stain moldings.
Another very serious problem is that the dimethylsiloxane oligomer may undergo oxidation, producing a silicon dioxide powder that can adversely affect surrounding electrical/electronic components. In the specific case of xerographic copiers, the silicon dioxide powder produced by dimethylsiloxane oligomer oxidation sticks to the corotron, thereby preventing production of clear, crisp copies.
In the case of continuous industrial processes that use solvent-based paint, the binder resin is cured while evaporating the solvent by heating and the gaseous by-products are completely combusted using a platinum catalyst. However, widen an amino-containing diorganopolysiloxane is present in the solvent-based paint, the silicon dioxide powder coats the surface of the platinum catalyst honeycomb, thereby impairing the efficacy of the catalyst.
As the result of extensive investigations directed at solving the aforementioned problems associated with prior art amino-functional diorganopolysiloxanes, the present inventors discovered that the problems can be eliminated by using mixtures of amino-functional diorganopolysiloxanes and amine-free dimethylpolysiloxanes exhibiting specified limits on amine equivalent weight, viscosity, and concentration of dimethylsiloxane oligomers containing up to 20 silicon atoms, represented in this specification by the expression Si.sub..ltoreq.20.
Amino-functional diorganopolysiloxanes have heretofore generally been prepared by the equilibration polymerization of a mixture of at least one cyclic dimethylsiloxane oligomer and at east one cyclic amino-containing diorganosiloxane oligomer in the presence of alkali catalyst, followed by removal of the low boiling point materials in the reaction product by heating under reduced pressure. A disadvantage of this method is that the Si.sub..ltoreq.20 dimethylsiloxane oligomers present in this reaction product have high boiling points, and their removal, even by heating under high vacuum, is quite problematic. Moreover, heating at excessively high temperatures can decompose the amino group. As a result, removal of this oligomer is all but impossible, which explains why prior amino--containing diorganopolysiloxanes have contained from 50,000 to 200,000 ppm of these undesirable dimethylsiloxane oligomers.
When Si.sub..ltoreq.20 amino-containing cyclic diorganopolysiloxanes are converted into a thin film and distilled by heating under a high vacuum, the content of Si.sub..ltoreq.20 dimethylsiloxane oligomers can be reduced to below 50,000 ppm. In the case of dimethylpolysiloxanes, after their preparation by continuous polymerization the content of Si.sub..ltoreq.20 dimethylsiloxane oligomer can be reduced to below 100 ppm by conversion into a thin film and distillation at elevated temperatures using high vacuum.